With regard to the continuous castings of carbon steels, low alloy steels, specialty steels and so on, more than twenty years have passed since the present vertical-bending-type continuous casting machines began to operate. And it has been said that these technologies became mature. On the other hand, the demand for the quality is increasing its severity year after year and the pressure to the cost-down also is increasing simultaneously. Aside from the problems such as breakout, etc. that often became a problem in the early period of the operating history, there still remains (1) central segregation and (2) central microporosity as the major problems concerning the quality.
The central segregation is the segregation having V characters that takes place with a periodicity in the middle of the thickness in the final solidification zone, and is generally called V segregation. The central microporosity is the microscopic void that forms in an interdendritic region also in the middle of the thickness in the final solidification zone. Summarizing these defects, they are to be called the central defects(internal defects) thereafter in this specification.
Next, the effects of the central defects on the quality of the steel products are briefly stated as follows.
(1) The Case of Thick Plate:
Hydrogen coagulates and precipitates into these central defects, and so-called hydrogen induced cracking results during usage. Also, upon welding, the weld cracking occurs starting these defects.
(2) The Cases of Rod and Wire:
Cracking takes place starting the microporosity during drawing.
(3) The Case of Thin Plate:
Upon pressing or during cold rolling, banded defects form, which result from the irregularity in hardness. This irregularity is caused by the coexistence of hard and soft spots due to segregation. The above defects takes place during the solidification process of continuous casting and lead to a poor quality product. The segregation formed during the solidification process remains in final products and can not be eliminated. Tentatively, there is a method of eliminating the macrosegregation by diffusion heat treatment. However, this method is not favorable both economically and technically because a long period of heat treatment at a high temperature is required. As for the microporosity, it is possible to smash them by hot rolling. But whether or not it can completely eliminate them depends on the quantity of the porosity. Furthermore, an attention must be paid to the fact that the microporosity accompany segregation in many cases.
Like this, the central defects is the problem associated with the essence of solidification phenomena, and the present situation is that it is very difficult to solve by means for the accumulation of know-how or by means for trials and errors based improvement. Although there is some difference in degree, these central defects(internal defects) are common to all the steel grades of slabs, blooms and billets. They exist from the beginning of the continuous casting history: Thus, they are an old but at the same time a new problem.
Among the measures that have been carried out until now to improve the internal defects, several important technologies will be reviewed in the following.
(1) Prevention of the Bulging
It has been said that the central segregation is formed in slabs with broad width when the solid part of the solidifying shell or the cast piece between supporting roll pitches was bulged by internal pressure of the steel melt. Although this happens by the flow of high solute concentration liquid within the solid-liquid coexisting zone which is induced by the deformation of the solidifying shell, the detailed mechanism is not clarified satisfactorily. Therefore, to reduce the bulging to as small extent as possible, such measures as shortening the roll pitches or dividing one roll into sub-rolls in the longitudinal direction have been employed. Besides, the misalignment of rolls is said to be responsible for an interdendritic fluid flow, thus causing the segregation. However, the internal defects can not be eliminated even if these mechanical disturbances are removed, considering the fact that the central segregation occurs even in the blooms and billets where the bulginess hardly become the problem.
(2) Strengthening of Secondary Cooling(Please Refer to Refs. (1) and (2) at the End of this Specification)
This is the method of intensively cooling the vicinity of the final solidification position (the crater end) to compress the solid-liquid coexisting phase by contraction force due to thermal stress so as to compensate the solidification contraction, thereby reducing central porosity. It has been reported according to the Refs. (1) and (2) that the improvement was made to some extent.
On the other hand, the main stream at present is the method of compressing the solidifying shell and give compressive deformation to central solid-liquid coexisting phase in the vicinity of final solidification position to control the interdendritic fluid flow, thereby reducing the internal defects. This method is divided into soft-reduction and hard-reduction depending on the amount of reduction.
(3) Soft-Reduction Method at the Last Stage of Solidification(Please Refer to Refs. (3) and (4))
With this method to improve the central segregation, the solid-liquid coexisting zone is compressed to compensate the solidification contraction which takes place continuously with the progress of solidification. With respect to the soft-reduction amount, a slope needs to be attached so as to correspond to the continuously arising solidification contraction as precisely as possible. For example, it is shown in Ref. (3) that the central segregation was improved by the real machine test of a carbon steel bloom that used the compressive crown roll with roundness attached. Also, in Ref. (4), examples are shown about theoretical estimates of reduction gradient necessary for the case of high carbon steel blooms (0.7 to 1 wt % C) with 300.times.500 mm section. According to the estimates, the gradient of 0.2 to 0.5 mm/m is required. However, various problems must be overcome to realize this method on a real machine, which will be stated below.
1 Usually, the soft-reduction is carried out in the range of a couple of meters in the vicinity of the final solidification zone, which becomes approximately 0.3 mm/m in the case of the blooms of the above Ref. (4).
This means that the inclination of 0.3 mm per 1m needs to be attached to the solidifying shell. Thus, the reduction amount must be controlled with great accuracy by means for a multi-rolled reduction apparatus, etc.
2 There is a difficulty that if the amount of reduction is not enough, the effect can not be expected, and that if the amount is excessive on the contrary, the interdendritic liquid flows backward to the upstream resulting in the channel segregation (i.e. inverse V segregation).
3 Required amount of reduction differs depending on the operating conditions such as a steel grade, dimensions in cross section, casting speed and cooling condition. Therefore, a great amount of labor and cost is necessary for these trials and errors to find an appropriate condition even in the case of a few steel grades.
4 Since the soft reduction method often gives rise to the new problem of internal cracking(Ref. (5)), a consideration must be taken into to prevent this.
Thus, it is not easy to make use of this method to achieve the purpose.
(4) Continuous Forging Method (Refs. (7) and (8))
Next, stated is the hard-reduction method in which the solid-liquid coexisting phase in the vicinity of the final solidification zone is mechanically largely deformed thereby squeezing the high solute concentration liquid to the upstream to prevent the central segregation (V segregation). There are two methods in this: One is to use large diameter rolls (Ref. (6)), and the other is the continuous forging method in which the shell is continuously forged using anvils (Refs. (7) and (8)). Because both belong to the same category in their concepts, only the latter is described in the following. As shown in FIG. 42, the vicinity of the final solidification zone is forged by anvils while moving toward casting direction together with the anvils. It has been reported that by repeating this cyclically, the high solute concentration liquid within the solid-liquid coexisting zone is squeezed into the upstream region with low volume fraction of solid, thus enabling it possible to suppress the central segregation. Also, it is said that the internal cracking can be avoided by setting up an appropriate forging condition. It is possible to control the segregation ratio Ke (=C/C.sub.0, C=average solute concentration, C.sub.0 =solute content) to be Ke&lt;1 by changing the volume fraction of solid at the time of forging.
The most important point of this method is to clarify the flow phenomenon in the solid-liquid coexisting zone at the time of forging. However, the authors of this reference have derived the relationship between segregation ratio Ke and the volume fraction of solid at the time of squeezing taking into account only the conservation law of solute elements. In their model, the liquid flow in the solid-liquid coexisting zone has not been treated explicitly, that is to say, the influences of the flow of solute concentrated liquid in the dendritic scale on the segregation has not been clarified. Therefore, while it is controllable as for the average macrosegregation in a macroscopic inspection range of the solid-liquid coexisting zone, the information about so-called semi-macrosegregation in much smaller inspection range (dendritic scale) can not be obtained. The semi-macrosegregation remains to some degree in their method.
Accordingly, the mechanism of the formation of the semi-macrosegregation belongs to a future subject and the flow phenomenon of the ejected liquid phase needs be clarified. In this connection, there is a possibility that the V segregation has already been formed when forged: in this case, the questions are raised on how the ejected liquid behaves, on if it remains as the semi-macrosegregation, etc.
In the above references, the blooms having nearly square cross section have been studied where the shape of solid-liquid coexisting zone can be approximated by a cylindrical form, and so when the solidifying shell is compressed in an iso-concentric fashion, the flow pattern will become comparatively simple. But in the case of slabs having broaden width, it is questionable whether or not the flow becomes a simple upstream pattern. In any case, it is not easy at all to predict the flow pattern of the solute concentrated liquid and to evaluate its influences when the solid-liquid coexisting zone is mechanically deformed to a large extent.
(5) Electromagnetic Stirring (Refs. (9) and (10))
This is the method of stirring the solid-liquid coexisting zone by an electromagnetic force in the vicinity of the final solidification zone to disperse the central segregation (Ref. (9)). For example, there is a method of spiral-stirring within the cross section of a solidifying shell. Another method is that the electromagnetic force is applied within the secondary cooling zone or within the mold with the aim of transiting columnar structure to equiaxed structure. The latter method is based on the prerequisite that the equiaxed structure is preferred to the columnar structure as for the central segregation, but the theoretical basis is poor. These methods are not an essential solution and are not the mainstream at present.
(6) The Combination of the Above Methods (1) to (5)
The bulging prevention measure has been esteemed consistently until the present as a fundamental technology and the following combinations are carried out based on this.
For example, it has been reported for carbon steel slabs of 0.08 to 0.18 wt % C in Ref. (10) that with the combination of short roll pitch with sub-segmented rolls (bulging prevention), taper-alignment method (gradually narrowing the gaps between the rolls in the downstream direction to correspond to the contraction of the cast piece due to solidification contraction and temperature drop--)--and strengthened cooling+electromagnetic stirring in the secondary cooling zone, the central segregation was improved compared to the case with no measures taken.
It is also stated in Ref. (11) that the central porosity reduces when the equiaxed structures are developed by simultaneously adopting low casting temperature and electromagnetic stirring for the carbon steel blooms and round billets in which the equiaxed structures are difficult to develop. Furthermore, it is reported that the central segregation and porosity can be reduced by adjusting the reduction amount in the final solidification zone and by developing the equiaxed grains via electromagnetic stirring within the mold.
(7) Cast Rolling Method in a Thin Slab Casting
So-alled mini-mill, that concisely sums up a steel making plant, has become increasingly popular because of such advantages as recycling of raw materials, the energy saving, a low plant construction cost and the gentleness to the earth environment in comparison with a large scaled conventional blast furnace.
With the mini-mill, thin slabs with the thickness of 50 mm or 60 mm (so-called near-net-shape-castings made as close to the size of the final products as possible) are cast, instead of large sectioned conventional castings with the dimensions of 200 mm or 300 mm.
Here, The Cast Rolling method (Ref. (12)) will be described as an example. This method is characterized by gradually compressing and thinning solidifying shell (reduction ratio of 10 to 30%) by rolls the range including the solid-liquid coexisting and liquid phases. This method is supposed to be born from an idea that the solidifying shell can be reduced during solidification considering that there is a limit to make thin at inlet nozzle position, by which the following effects are reported.
1. Because dendrites are mechanically destroyed, equiaxed fine grains are formed.
2. As a result, the macrosegregation is fairly decreased.
However, the flow behavior of high solute concentration liquid induced during heavy deformation of the solid-liquid coexisting zone is unpredictable, and therefore it is not easy to control so as to avoid detrimental defects such as inverse V segregation, etc.
So far, key technologies to improve the internal quality of continuous castings of steels were reviewed from a vast amount of literature. Historically speaking, they trace back to the taper-alignment method for the control of the bulging that causes segregation, progress into the shortened roll pitch/divided roll method, strengthened secondary cooling, electromagnetic stirring and presently become soft/hard-reduction methods or the combination of the electromagnetic stirring and the soft-reduction. Although the technology has been improving meanwhile, it has not yet reached to the essential solution of the problem.